Known methods for forming a gallium oxide-based thin film having high crystallinity on a sample to be deposited include a film forming method using water fine particles, such as the mist CVD method (Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2013-28480). For example, this method is used as follows: a gallium compound such as gallium acetylacetonate is dissolved in an acid such as hydrochloric acid to prepare a raw-material solution; the raw-material solution is atomized into raw-material fine particles; the raw-material fine particles are carried by a carrier gas onto the film forming surface of the sample; and the raw-material fine particles being in a mist state is caused to react to form a thin film on the film forming surface. Thus, a gallium oxide-based thin film having high crystallinity is formed on the sample.
To form a semiconductor device using a gallium oxide-based thin film, it is required to control the electrical conductivity of the thin film. Patent Literature 1 and Non-Patent Literature 1 [Electrical Conductive Corundum-Structured α-Ga2O3 Thin Films on Sapphire with Tin-Doping Grown by Spray-Assisted Mist Chemical Vapor Deposition, Japanese Journal of Applied Physics, 51 (2012) 070203] disclose technologies for doping an α-gallium oxide thin film.